Do backlog students get placed in summer internship?

From what I have heard and read, yes they do get placed.
However, it depends from company to company. Investment banks and consulting firms lay emphasis on your UG CGPA. A lot of companies focus more on other aspects of your CV.
If questioned in an interview, one must clearly justify the slack in performance, and how it is not a reflection on your candidature for a particular company.

Yes, definitely!
The students who repeat their first year in an MBA program go through the same process all over again. So, they are allowed to sit for the Summer Placements and also have equal opportunities. Additionally, if the CV of the student is strong enough, and reflects substantial profile as required for the job, he/she will get the shortlist.

As long as your CG is beautiful and your CV has what the company is looking for your backlogs in undergrads won’t matter. Even people who repeat their 1st year in MBA get placed in good companies if their CV is strong. So at the end it boils down to what compensates your CG in your CV.